IC221: Systems Programming
6-Week Written Exam
[SOLUTIONS]
February 12, 2014
Answer the questions in the spaces provided on the question sheets. If you run out of room
for an answer, continue on the back of the page. Show all work, but please be legible.
Microscopic writing will not be graded. This is a closed exam. No additional material may be used
to complete this exam.
Name:
Alpha:
Question
Points
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15
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15
3
15
4
15
5
15
Total:
75
Score
IC221
6-Week Written Exam
February 12, 2014
1. For the following questions, assume a file system with the following structure. All directories end in /,
all files have no set file extension, and ∼ is the home directory.
~/
|
| - - > foo /
|
| ___ . - >
|
| ___ . - >
|
| - - > bar /
|
| ___ . - >
|
| ___ . - >
|
| ___ . - >
|
| - - - > baz /
bar . txt
foo
bar . jpg
foo
baz /
(a) [3 points] What is the output of running ls from the home directory?
foo bar baz
[+1 for each]
(b) [3 points] What is the output of running the command ls -a bar from the home directory?
. .. bar.jpg foo baz
[+2 for each named file, +1 for the . and ..]
(c) [3 points] Write a glob pattern that will list, with ls, all files that start with bar. Assume that
the ls is run from the home directory.
ls */bar.*
[+2 bar.* and +1 for having */]
(d) [3 points] Write a find command that will find all directories and print them to the terminal.
Assume that the find is run from the home directory.
find . -type d
[+3 for -type d, +1 for -type with no d]
(e) [3 points] Write a find command with an xargs or -exec that will remove all files (not directories)
named foo.
find . -type f -name foo -exec rm {} \;
OR
find . -type f -name foo | xargs rm
[+2 for -type f and -name foo, +1 for rm with exec or xargs]
Page 2 of 6
IC221
6-Week Written Exam
February 12, 2014
2. For this question, assume you have a comma separate file (database.csv) with the following fields:
First Name , Last Name , Street Address , City , State , Phone Number
The state, is a two-character postal code (e.g. MD for Maryland and CA for California). The phone
number is always formatted like 999-999-9999. The database.csv is not corrupted, and none of the
fields may have a comma in it.
(a) [3 points] Write a command line to count the number of unique states in database.csv. Your
command may contain pipes and redirects.
cut -d "," -f 5 | sort | uniq | wc -l
[+2 for good cut, +1 for sort, unqiq and a wc]
(b) [3 points] Write a command line to extract all the area codes of each of each of the phone numbers
in database.csv. It is fine if the same area code is printed multiple times. Your command may
contain pipes and redirects.
cut -d "," -f 6 | cut -d "-" -f 1
[+2 for one cut, +1 for other cut]
(c) [3 points] Write a command line that will save all lines in database.csv that contain the state
code “MD”, for Maryland, anywhere on a database line. Then save those lines to a file named
database.md.csv. Your command may contain pipes and redirects.
grep "MD" database.csv > database.md.csv
[+2 for either grep or redirect alone, +3 for both parts together, +1 for just writing grep]
(d) [3 points] What are the three guiding principles of the Unix design philosophy?
(1) Write programs that do one thing and do that one thing well
(2) Write program that work well together
(3) Write programs to handle text streams
[+1 for each principle]
(e) [3 points] Using an answer from above, describe how that command meets the Unix design philosophy.
[+1 for using term pipeline, +1 for reefering to a prior command, and +1 for covering each of the
principles.]
Page 3 of 6
IC221
6-Week Written Exam
February 12, 2014
3. The questions below all reference this small program:
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int main (){
int fd , n ;
char buf [4096];
if ( ( fd = open ( " save . out " , O_WRONLY | O_TRUNC | O_CREAT , 0644)) < 0){
fprintf ( stderr , " ERROR : Cannot Open File \ n " );
return 2; // error !
}
while ( ( n = read (0 , buf , 4096)) > 0 ){
if ( write ( fd , buf , n ) < 0 ){
fprintf ( stderr , " ERROR : Bad Write \ n " );
return 2; // error !
}
}
if ( n < 0 ){
fprintf ( stderr , " ERROR : Invalid Read \ n " );
return 2;
}
close ( fd );
return 0;
}
(a) [4 points] In the code above, place a circle around all library functions and a box around all
system calls. What is the purpose of a system call with respect to the user interaction with the
operating system?
Library Functions: fprintf()
System Calls: open(), read(), write(), close()
A system call is a request to the O.S. to perform a privileged operation.
[+2 for circling/boxing, +1 for explanation without “privileged operation”, +2 for with]
(b) [3 points] In the open() function call on Line 5, use the following option flags: O WRONLY, O TRUNC,
O CREAT. Describe in plain English what each of these flags mean and the method that is being used
to combine them.
Open for writing, truncate if it exists, and create if it doesn’t. Options are combined using an ORing.
[+2 for explantation, +1 for ORing]
(c) [4 points] What are the file descriptor numbers and names for each of the standard file descriptors?
Which of those are being used in the above program?
0: Standard Input, 1: Standard Output, 2: Standard Error
0, standard input, is used in the read(), stderr is used in fprintf()
[+1 for each descriptor, +1 for which is used]
(d) [4 points] What does the above program do?
Reads from standard input and saves results in file save.out, just like from lab 3. [+2 for reads from standard
input, +2 for saves whats read to file save.out. Provide credit for other explanations of the open and
error process, if given, but not full credit without the top 2.]
Page 4 of 6
IC221
6-Week Written Exam
February 12, 2014
4. The questions below all reference the following bash script, foobar existence.sh:
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# !/ bin / bash
for arg in $ *
do
if [ $arg == " foo " ]
then
echo " You say foo , I say bar "
else
ls $arg >/ dev / null 2 >&1
res = $ ?
if [ $res - eq 0 ]
then
echo " $arg exists "
else
echo " $arg DOES NOT exist "
fi
fi
done
(a) [3 points] On line 1, there is a special kind of comment to start the bash script. What does this
comment do with respect to running the script in the terminal like, ./foobar existence.sh?
The shabang, or pound-bang, and it tells the terminal how to interpret the script, such as to treat this as bash.
[+1 for shabang or # bang, +2 interpret or treat as bash when executed]
(b) [3 points] What values do the special bash variables $* and $? store? Refer to the script to
explain.
$* stores the arguments, iterate through args in for loop, and $? stores result of operation, such as after the ls.
[+1 for each explanation, +1 for referring to the script in some way]
(c) [4 points] On line 9, there is a sequence of redirects with the ls command. What standard file
descriptors are redirected where?
Standard error gets redirect to standard out which all get’s redirected to /dev/null
[+2 for stderr to stdout, +2 for stdout to /dev/null]
(d) [5 points] Using the directory structure from Question 1, what is the output of running foobar existence.sh
from the home directory with the following arguments: foo bar/foo xyzz bar/bar.jpg.
You say foo, I say bar
bar/foo exists
xyzz DOES NOT exist
bar/bar.jpg exits
[+2 for “you say foo ...”, +1 for each of the others]
Page 5 of 6
IC221
6-Week Written Exam
February 12, 2014
5. Consider the following C code:
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typedef struct {
int left ;
int right ;
} pair_t ;
int main (){
pair_t pair_array [4];
pair_t * p ;
int i ;
for ( i =0; i <4; i ++){
pair_array [ i ]. left = i ;
pair_array [ i ]. right = i *2;
}
p = pair_array +2; /* MARK */
printf ( " A : (% d ,% d )\ n " , p - > left , p - > right );
printf ( " B : (% d ,% d )\ n " , p [1]. left , p [1]. right );
return 0;
}
(a) [4 points] Draw the stack/memory diagram for the above code at the MARK.
[+2 for array layout, +2 for p pointing]
(b) [4 points] What is the output of this program?
A: (2,4)
B: (3,6)
[+2 for each]
(c) [4 points] What is the difference between the . and the -> operators when accessing structure
members? Use the above code in your explanation.
The . operator is for when you working with the structure directory, and the -> is when you have a pointer to the
structure, require a deference.
[+2 for each description]
(d) [3 points] Does any memory need to be de-allocated in this program? If so, what? And, if not,
why not?
No, everything is declared on the stack. No malloc().
[+3 for No with explanation, +2 for No with no explanation, +1 for wrong answer with explanation]
Page 6 of 6